Electrical conduction between brushes and slip rings have long encountered problems relating to frictional drag, noise, electrical resistance and even open circuits due to chemical film or physical fouling of contact points by foreign debris. These problems, particularly open circuits, have been magnified with miniaturization.
Electrical conduction through mechanically mating contact members does not occur over a continuous or substantial area. All fabricated structures have some degree of surface roughness, and when two such surfaces are brought into mechanical contact they actually touch only where their peaks (asperities) on opposing topographies coincide to a define "a" spots. As contact force on the structures is increased, the asperities are deformed, thus increasing individual "a" spot areas, and allowing other asperities to begin making contact. In practice, increasing contact force does not ultimately achieve total area contact.
The flow of electrical current from one contact member to another is constrained to mating "a" spots, thereby giving rise to a constriction resistance due to the decreased conductive area in the contact interface. In addition to constrictive resistance, there are always, except in perfect vacuum, films of various types which contribute additional resistance. These films may cause a contact to perform poorly, or fail altogether. Relative movement between the members cause wear at their interfacing "a" spots. Some beneficial effects of wear are that mating contacts become fitted to each other, thereby smoothing the surface and increasing contact area. A deleterious effect is that the wear creates debris, causing intermittent performance and electrical shorts between adjacent circuits.
These factors must be taken into consideration in the design of miniaturized precision slip rings and brushes for inclusion on gyroscopes used in inertial guidance systems where frictional drag, noise, resistance to current flow, and an environment free of foreign bodies are important considerations. In considering loop brush designs for miniaturized gyroscopes there are trade-offs in brush wire sizes, brush tension, noise, wear, and angle of brush wrap about the slip ring.
One design of a miniaturized slip ring and brush design is disclosed in U.S. Pat. No. 3,396,586. While this patent is primarily concerned with a method of assembling slip rings with a plurality of axially-spaced brushes, it discloses slip rings being contacted by a plurality of axially spaced brushes which are pieces of spring tempered round wire sections bent to a U-shape, the legs of which make point contact on opposite sides of each slip ring.
Typical wire brush arrangement in the prior art employ U-shaped round leg brushes riding in V-groove slip rings which provided a maximum of four junctures per circuit. It is obvious that contact spots which are commonly only a few micrometers in diameter are quite susceptible to contamination. Airborne dusts, for example, are common in size range of one to 20 micrometers, and bacteria may be about one micrometer in diameter. Other physical interference by free bodies, such as from abraded parts and organic vapors, add to the problem of maintaining electrical contact through the "a" spots.